fix make docsgen-cli use python3

[snissn]

Related Issues

make docsgen-cli was failing on mac

Proposed Changes

changing Makefile to explicitly call python3

Additional Info

we're discussing rewriting ./scripts/generate-lotus-cli.py into go to not require python. However since we already are compatible with python3 and python 2 is not supported anymore, the explicit change to python3 is good. Simialrly once we rewrite generate lotus cli in GO, it's not bad that we've accepted this change, we can then still rip out python, but in the meantime having it supported on more. machines is good.

Checklist

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